Use IsDeltaPayload instead of IsDelta.

Trying to resurrect the IsDelta in an earlier CL to avoid pollution of
Omaha rules turned out to be a wrong choice because R14 and R15 code
strictly expects IsDelta to be true even for full payloads. So
preserving IsDelta's semantics as-is and introducing IsDeltaPayload to
really convey the intended semantics: True if it's delta payload, False
if it's full payload.

BUG=chromium-os:37801
TEST=Updated unit tests. Works fine on ZGB with Omaha devserver.

Change-Id: I899b56be4dcbdcf1981f72d356d40a1e99fdf725
Reviewed-on: https://gerrit.chromium.org/gerrit/41181
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Jay Srinivasan <jaysri@chromium.org>
Tested-by: Jay Srinivasan <jaysri@chromium.org>
diff --git a/omaha_request_action.cc b/omaha_request_action.cc
index edfea53..e994335 100644
--- a/omaha_request_action.cc
+++ b/omaha_request_action.cc
@@ -34,7 +34,8 @@
 static const char* kTagDeadline = "deadline";
 static const char* kTagDisablePayloadBackoff = "DisablePayloadBackoff";
 static const char* kTagDisplayVersion = "DisplayVersion";
-static const char* kTagIsDeltaPayload = "IsDelta";
+// Deprecated: "IsDelta"
+static const char* kTagIsDeltaPayload = "IsDeltaPayload";
 static const char* kTagMaxFailureCountPerUrl = "MaxFailureCountPerUrl";
 static const char* kTagMaxDaysToScatter = "MaxDaysToScatter";
 // Deprecated: "ManifestSignatureRsa"